1. Field of the Invention
The present invention relates to piezoelectric oscillators in which piezoelectric resonators and capacitors are combined. More particularly, the present invention relates to piezoelectric oscillators having piezoelectric resonators connected to load capacitances and parallel capacitances.
2. Description of the Related Art
Conventionally, piezoelectric oscillators have been widely used to obtain oscillation frequencies for clock circuits or other suitable devices. In such piezoelectric oscillators, outstanding frequency accuracy and reliability are required.
Japanese Unexamined Utility Model Application Publication No. 5-18120 provides a piezoelectric oscillator. FIG. 7 shows a sectional view of the piezoelectric oscillator. In this figure, a piezoelectric oscillator 101 includes a piezoelectric resonator 103 mounted on a capacitor substrate 102. The piezoelectric resonator 103 is enclosed by a cap 104. External electrodes 102a to 102c are provided on outer surfaces of the capacitor substrate 102. The external electrodes 102a and 102b are provided on each end surface of the capacitor substrate 102. The portions of the external electrodes 102a and 102b that extend to the upper surface of the capacitor substrate 102 are electrically connected to vibration electrodes 103a and 103b of the piezoelectric resonator 103.
Internal electrodes 102d and 102e are provided inside the capacitor substrate 102. The internal electrodes 102d and 102e are arranged to overlap each other via a dielectric layer. The internal electrode 102d is connected to the external electrode 102b and the internal electrode 102e is connected to the external electrode 102a. The external electrode 102c extends from the upper surface of the capacitor substrate 102 to the lower surface through a pair of side surfaces of the substrate 102. In addition, the external electrode 102 overlaps with the external electrode 102d and 102e via a dielectric layer.
With the above arrangement, a parallel capacitance Cp is provided between the external electrodes 102a and 102b based on the dielectric layer between the internal electrodes 102d and 102e. In addition, a load capacitance C11 is provided between the internal electrode 102d and the external electrode 102c and a load capacitance C12 is provided between the internal electrode 102e and the external electrode 102c. FIG. 8 shows the circuit structure of the piezoelectric oscillator 101.
In the piezoelectric oscillator 101, the parallel capacitance Cp is connected in parallel to the piezoelectric resonator 103. Thus, by adjusting the magnitude of the parallel capacitance Cp, a spurious response occurring between a resonance frequency and an anti-resonance frequency can be shifted. As a result, spurious responses occurring on the high frequency side can be suppressed.
Furthermore, Japanese Unexamined Patent Application Publication No. 4-192709 provides a piezoelectric oscillator, which is shown in FIG. 9. In a piezoelectric oscillator 111, a piezoelectric resonator 113 is mounted on a capacitor substrate 112. The piezoelectric resonator 113 is enclosed by a cap 114. A plurality of internal electrodes 112a to 112e are provided inside the capacitor substrate 112. The internal electrodes 112a and 112b oppose each other via a predetermined distance at the same height position. The internal electrode 112a is connected to a via-hole electrode 112f and the internal electrode 112b is connected to a via-hole electrode 112g. Similarly, the internal electrodes 112d and 112e oppose each other at the same height position and the internal electrode 112d is connected to the via-hole electrode 112f and the internal electrode 112e is connected to the via-hole electrode 112g.
The internal electrode 112c is arranged to overlap the internal electrodes 112a, 112b, 112d, and 112e via dielectric layers. The internal electrode 112c is connected to a via-hole electrode 112h. The via-hole electrodes 112f to 112h extend to the lower surface of the capacitor substrate 112. External electrodes 112i to 112k are provided on the lower surface of the capacitor substrate 112 to be electrically connected to the outside. The external electrodes 112i to 112k are connected to the via-hole electrodes 112f to 112h. 
In the piezoelectric oscillator 111, the internal electrodes 112a to 112e are provided inside the capacitor substrate 112, and a load capacitance is thereby connected to the piezoelectric resonator 113. In this oscillator 111, since the external electrodes 112i to 112k are provided on the lower surface of the capacitor substrate 112, the lower-surface side of the capacitor substrate 112 can be mounted on a printed circuit board.
Recently, in such a piezoelectric oscillator, as in other kinds of electronic devices, miniaturization, and particularly, height reduction has been strongly demanded. When a piezoelectric oscillator is miniaturized, the size of a piezoelectric resonator used in the oscillator is also reduced. As a result, a vibration-electrode facing area in the piezoelectric resonator is reduced. Consequently, a capacitance Cf between terminals in the piezoelectric resonator is reduced. For example, when the piezoelectric resonator is made of a ceramic material including Pb having a high Q factor, since the relative bandwidth is broadened, the frequency accuracy varies due to variations in the magnitudes of external capacitances with respect to the piezoelectric resonator.
As the piezoelectric ceramic defining the piezoelectric resonator becomes finer and stronger, the high frequency characteristics are greatly improved. However, with the high frequency characteristics improved, high-order mode spurious responses such as a third harmonic and a fifth harmonic causing abnormal oscillation are substantially increased. Thus, such high order mode spurious responses must be suppressed.
In addition, as the height of the oscillator is reduced, the thickness of a gap between the piezoelectric resonator and the capacitor substrate on which the piezoelectric resonator is mounted is reduced to a few xcexcm. As a result, for example, in the oscillator as described in Japanese Unexamined Utility Model Application Publication No. 5-18120, when the piezoelectric resonator 103 resonates, the vibration electrodes which extend on the lower surface of the piezoelectric resonator 103 come in contact with the external electrode 102b connected to a ground potential on the capacitor substrate and are eventually short-circuited.
In the piezoelectric oscillator 111 described in Japanese Unexamined Patent Application Publication No. 4-192709, no external electrode is provided on the upper surface of the capacitor substrate 112. Thus, when the height of the oscillator is reduced, the above-mentioned short circuit does not occur. However, in the piezoelectric oscillator 111, as the size of the oscillator is reduced, as described above, the influences caused by the variations in external electrodes are greater. Thus, it is very difficult to improve the frequency accuracy. In addition, the influences of high order mode spurious responses including a third harmonic and a fifth harmonic are increased.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide an improved piezoelectric oscillator. In this piezoelectric oscillator, since the deterioration of the frequency accuracy caused by variations in external capacitances is minimized, excellent frequency characteristics are obtained. Moreover, even when the height of the oscillator is reduced, undesired short circuits do not occur. Thus, the piezoelectric oscillator of preferred embodiments of the present invention has outstanding accuracy and reliability.
In addition to this, in the piezoelectric oscillator of preferred embodiments of the present invention, even when high frequency characteristics are greatly improved, higher order mode spurious responses causing abnormal oscillation are effectively minimized.
According to a preferred embodiment of the present invention, a piezoelectric oscillator includes a capacitor substrate having a first main surface and a second main surface, a pair of side surfaces, and a pair of end surfaces, first and second external electrodes including portions which extend to the first main surface of the capacitor substrate, a third external electrode which does not extend to the first main surface, a piezoelectric resonator mounted on the first main surface and electrically connected to the first and second external electrodes, a parallel capacitance section connected between the first and second external electrodes to load a capacitance in parallel to the piezoelectric resonator, and a pair of load capacitance sections for interposing a load capacitance between the first and third external electrodes and a load capacitance between the second and third external electrodes, in which the parallel capacitance section and the pair of load capacitance sections are contained inside the capacitor substrate.
In this oscillator, as capacitance-frequency characteristics of the parallel capacitance section are present in a higher frequency position, the value of tanxcex4 is greater.
In addition, the parallel capacitance section and the pair of load capacitance sections are preferably defined by a plurality of internal electrodes overlapping each other in the thickness direction of the capacitor substrate via dielectric layers defining the capacitor substrate.
In addition, the relative permittivity of the dielectric layer in the parallel capacitance section differs from the relative permittivities of the dielectric layers in the load capacitance sections.
Additionally, the piezoelectric oscillator further includes a cap attached to the first main surface of the capacitor substrate to enclose the piezoelectric resonator mounted on the first main surface.
Furthermore, this oscillator preferably further includes a frame-shaped insulation layer provided on the first main surface of the capacitor substrate to mount the piezoelectric resonator.
In addition, the first and second external electrodes extend from the first main surface of the capacitor substrate to the pair of side surfaces thereof, and the third external electrode is provided on the pair of side surfaces.
In addition, in this oscillator, the first to third external electrodes preferably extend to the second main surface of the capacitor substrate.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed explanations of preferred embodiments of the present invention with reference to the drawings.